1. Field of the Invention
The present invention relates generally to an in-line holographic lens arrangement which is particularly designed to produce and pass a symmetrical focused beam of light while blocking or attenuating the zero order, in-line collimated beam illuminating the lens. The subject invention is particularly applicable to high quality, low f-number holographic lenses.
2. Discussion of the Prior Art
Holographic lenses and multiple holographic lenses are well known in the art, and one particular application thereof, which is relevant to the present invention, is in the field of optical correlators.
In greater particularity, in such optical correlators an input image is correlated with optical information stored in a matched filter or multiple matched filters to provide identification and aspect information about the input image. In one particular design, an input image is directed onto a spatial light modulator to spatially modulate a coherent beam of radiation. The spatially modulated radiation beam is directed onto a multiple holographic lens which performs a multiple number of Fourier transformations thereon to obtain an array of a multiple set of Fourier transforms of the spatially modulated radiation beam. A corresponding array of matched filters has the array of Fourier transforms incident thereon, with each matched filter comprising a Fourier transform hologram of an aspect view of an object of interest. Each matched filter passes an optical correlation signal in dependence upon the degree of correlation of the Fourier transform of the spatially modulated radiation beam with the Fourier transform hologram recorded thereon. An inverse Fourier transform lens receives the optical correlation outputs of the array of matched filters, and performs an inverse Fourier transformation on each optical correlation output, which is directed to an output correlation plane. A detector at the output correlation plane then detects the optical correlation output, and produces a detector output signal representative thereof. A processing circuit compares the relative magnitudes of the signals to determine identification and aspect information about the input image.
A simpler form of such an optical correlator would utilize a single Fourier transform holographic lens and a single matched filter. Moreover, such optical correlators can employ off-line multiple holographic lenses, or in-line multiple holographic lenses, and the present invention is concerned only with the latter in-line optical arrangements.
However, it should also be stressed that the teachings of the present invention have general application to in-line optical systems employing lens elements therein, and are not limited only to applications in optical correlators as described hereinabove.
Moreover, the prior art dealing with holographic lenses can be categorized into in-line and off-line holographic lenses, and the present invention is generally limited only to in-line optical systems.
Kojima, U.S. Pat. No. 4,312,559 discloses the construction of an in-line holographic lens for use with optical signal reproduction systems, (e.g., optical discs, records, etc.). The desired lens is to have a large numerical aperture.
There are several differences between the present invention and this patent. First, Kojima's method involves large angle separation between the reference and focused object beam, which serves to suppress the conjugate order in the reconstruction process. This technique is effective only for thick emulsion holograms in which the Bragg effect is responsible for the inefficient reconstruction of the undesired component beam (note that the conjugate beam is much weaker than the DC beam which interferes with the focused beam). However, because of the large angle separation between recording beams, the recorded interference pattern is very asymmetrical. This asymmetry is responsible for the poor focusing qualities of such a holographic lens. This assymetrical disadvantage is a feature that the subject invention seeks to eliminate. Furthermore, the subject invention is not restricted to thick emulsions, and any type of emulsion can be utilized.
Secondly, the reconstructed zero order DC beam and the desired focused beam overlap in the patented system, and no attempt is made to separate the two beams. This is a serious problem, particularly for optical correlator applications, since the DC beam is comparable in intensity to features in the neighborhood of the focused beam, in the matched filter plane of the optical correlator. In contrast thereto, in the subject invention a primary objective is to avoid beam overlap by eliminating the DC beam.
Also, for reconstruction in the patented system, the hologram is illuminated by the reconstruction beam on the opposite side of the plate used for the recording exposure, which results in phase distortions introduced by the glass or film substrate. The present invention does not introduce similar phase distortions.
This patent also states in column 4, line 15 et. seq. that the reconstruction beam in the system must move up and down with respect to the hololens. Since the assymetrical hologram recording is being illuminated over different portions, the resulting focused beam will suffer distortions that are not constant. In contrast thereto, the present invention does not involve relative motion of the reconstruction beam, and consequently does not introduce such distortions.
Finally, in column 4, line 25, et. seq., the Kojima patent states that the in-line lens does not require difficult initial alignment adjustments. However, in this system the alignment of the reconstruction beam must still be maintained since it must move relative to the hololens plate, which is not a constraint in the present invention.